High power fiber laser system with high quality beam

ABSTRACT

A high power fiber laser system has a combiner configured of a plurality of single mode (SM) fibers which are fused together so as to define an output end of the fiber combiner. The fused SM fibers radiate respective fiber outputs, which collectively define a multimode (MM) combiner output. The SM fibers each are configured with such an optimally small numerical apertures (NA) that the MM combiner output is characterized by a minimally possible beam quality factor (M 2 ) for the plurality of SM fibers. To reduce the possibility of burning of the components of the fiber laser system with a multi-kilowatt combiner output, a coreless termination block is fused to the output end of the fiber combiner and configured so as to provide expansion of the combiner output without modifying the minimally possible M 2  factor thereof.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The invention relates to high power fiber laser systems. Particularly,the invention relates to a fiber combiner configured with a plurality ofSM fiber lasers which collectively radiate a combiner outputcharacterized by minimally possible M² factor.

2. Discussion of the Known Art

One of the most significant keys to ensuring broad marketability offiber lasers is the development of producing ever-increasing laseroutput powers without sacrificing beam quality. Known for itshigh-quality beam, a single-mode (SM) fiber laser may deliver outputpowers approaching ten kilowatts.¹ Despite the impressive results, evenhigher power levels and beam quality are required for current and futureindustrial and military needs. For example, SM fiber lasers developedand manufactured by IPG Photonics Corporation, Oxford, MA, U.S.A.

It is known to power scale laser output by combining the outputs fromseveral SM fiber lasers while sacrificing beam quality as the powerincreases. The combined single modes of respective fibers translate intoa multimode (MM) combiner output. The beam quality may be characterizedby an M² factor. The lower the M² factor, the higher the beam quality.In a diffraction-limited Gaussian beam, the M² factor is as low as one.The diffraction-limited beam is manifested by a single light spot. Insome applications, the beam quality factor is not critical, in others,which are of interest here, it is.

FIG. 1 illustrates the concept of divergence of a Gaussian beam radiatedby SM fiber. Quantitatively, the far-field divergence of the SM beam canbe measured as

$\begin{matrix}{\theta = \frac{2\lambda}{\pi\;{MFD}}} & (1)\end{matrix}$Wherein θ—divergence half-angle, MFD—mode field diameter. In single modefibers, the half angle is correlated to a numerical aperture as follows:sin θ=NA  (2)Accordingly, the divergence of the SM beam can be controlled by changingthe MFD, which is the waist diameter of the Gaussian beam in the SMfibers (FIG. 1).

FIG. 2, related to the above-disclosed equations, illustrates thedependence between the divergence or NA and MFD. The greater the MFD,the smaller the divergence. The MFD, in turn, depends on a core size Dcof SM fiber, as can be seen in FIG. 3. Seemingly, the limitless decreaseof the core diameter causes the increase of the MFD. In reality, the MFDcannot be limitlessly increased without detrimentally affecting the beamquality, as discussed below.

FIGS. 4 and 5 illustrate a known fiber laser system 10 configured withcombined multiple SM laser outputs 12 which are placed next to oneanother to form a combiner 14 with an output beam 16. Disclosed only asan example, seven parallel outputs of respective SM fiber lasers 12,each having a 125 μm outer diameter Df, define an effective area X ofoutput light beam 16 with a 375 μm overall outer diameter Db.

Since SM fibers 12 do not experience external stresses, beams 18,propagating in respective cores 20, each have a Gaussian shape. The MFDof each propagating beam is relatively small, and therefore, a far fielddivergence thereof is broad. Superimposed with one another, sevendistinct and spaced apart fiber outputs define combiner output 16 with alarge effective area X. The large effective area of the beam, i.e. animaginary boundary running around cladding of respective fibers 12,represents a broad far-field divergence or small numerical aperture and,therefore, a high M² factor of beam 16. In other words, system 10radiates a low-quality, relatively unfocused combiner output.

The fiber laser system 10 not only radiates an output beam of poorquality, but also the system is labor- and cost-ineffective. After thecombiner output diverges along seven fiber output paths, each fiberoutput is associated with a bulk optics (telescope) located along thefiber path. Only then seven fiber outputs converge toward one another.The necessity of seven additional bulk optic units adds labor efforts tothe manufacturing and tuning process and, therefore, may make thecombiner prohibitively expansive.

In practice, the terminal ends of respective fiber lasers 12, definingcombiner 14, are often processed to reduce the effective area of outputbeam 16. However, as far as Applicants know, there are no establishedmethods of controllably reducing the far-field divergence and M² factorfor active or doped SM fibers. The “blind” minimization of the effectivearea of the combiner beam, however, may not lead to satisfactoryresults, such a low M² factor and small divergence of combiner output,as discussed hereinbelow.

A need, therefore, exists for a method of controllably manufacturing afiber combiner operative to output a high power beam characterized bynarrow far-field divergence or small numerical aperture (NA) and low M²factor.

A further need exists for a high power laser system with the disclosedcombiner.

SUMMARY OF THE INVENTION

These needs are met by a high power laser system configured with aplurality of SM fiber lasers which are coupled together to define afiber laser combiner. The combiner is operative to radiate a combineroutput characterized by an optimally limited far-field divergenceselected so that the combiner output has a minimally possible M².

In accordance with one aspect of the disclosure, disclosed is a methodof configuring a fiber laser combiner having a low far-field divergenceand, thus, high quality combiner output. Initially, terminal ends ofrespective peripheral SM fiber lasers are arranged around a terminal endof central SM fiber so as to define a combiner. The combiner is furtherexposed to heat and tension making the terminal ends simultaneouslyelongate and radially shrink. The reduction is monitored so thatmultiple fiber outputs each maintain a substantially Gaussian shape.

The desired result of the disclosed process is to have a combiner outputhaving an optimally small far-field divergence which provides for thesmallest possible M² factor. In order to achieve these objectives, theeffective area of the combiner output, i.e., the cumulative radiation ofindividual SM fiber lasers should be as small as possible. The latter,in turn, is attained by minimizing individual fibers causing theminimization of both the numerical aperture (NA) of the individual fiberoutputs and the NA the combiner output. The process provides forcontrollably reducing the core diameters of respective SM fiber lasers;otherwise, the individual fiber outputs each would loose its high beamquality M². Thus, the controllable reduction of the fiber core diameterand, therefore, numerical aperture of each SM fiber laser allows for afiber output having optimally large MFD. Superimposed, a plurality ofthe fiber outputs define a small effective area of the multi-mode (MM)combiner output characterized by a minimally possible M² factor. Asreadily realized by the ordinary skilled worker, if a Gaussian shape ofindividual fiber outputs is maintained during the stretching of thecombiner, the measured NA of each individual fiber output and the NA thecombiner output are substantially the same.

As the cores of respective SM fiber lasers narrow, the divergence halfangle and, therefore, numerical aperture of each SM fiber laser iscontinuously measured so as to not exceed a reference value, asdisclosed below. Alternatively or in addition to the measurement of thehalf-angle of each fiber output, the half-angle of the combiner outputmay be measured. In either case, knowing the divergence of the fiberand/or combiner outputs, the M² factor can be determined. But theminimization of the NA aperture is not limitless because the mode ofeach fiber characterized by a mode field diameter (MFD) may expand intothe outer boundary of the cladding of the fiber output. The outerboundary of the combiner's output is formed as a result of heating andstretching of the SM fiber lasers whose outer boundaries of respectivecladdings gradually merge with one another to eventually define theouter boundary of the combiner output. Even if a single individual modereaches the outer boundary of the combiner output, the quality of thefiber output radically deteriorates. Accordingly, the combiner output ofthe disclosed combiner owns its high quality to the preservation of theGaussian shape of each individual SM fiber output having an optimallysmall NA and, thus, M² as well as an optimally large MFD which areattained in accordance with the disclosed process.

A further aspect of the disclosure relates to the safety and effectiveoperation of the above-disclosed combiner. As easily realized by theordinary skilled artisan, the disclosed laser system may reach tens andtens of kW because the number of SM fiber lasers may very well reach afew tens and each SM laser may be configured with increasingly highoutput power. The power density of the combiner output is extremely highand may easily damage outer coatings covering the combiner's outputfiber.

Furthermore, the use of the disclosed multi-kW laser system isassociated with powerful backreflected light from the end surface whilepropagating upstream of the system, can easily destroy the SM fiberlasers. Typically, the effective configuration preventing propagation ofbackreflected light includes the presence of anti-reflecting (AR)coating. However, technologically, it is difficult to polish thecombiner's output fiber so as to effectively apply the backreflectingcoating thereto. Moreover, even if the combiner's output is covered byan AR coating, the power density is so high that it can easily destroythe AR coating.

Both of the above difficulties can be substantially simplified byproviding a termination block configured with a quartz block which isfused to the downstream end of the combiner's output. The rationalbehind such a structure is simple. First, having the block with arelatively large inner diameter allows the combiner output beam toexpand. As a consequence, the power density of the combiner output isreduced and, therefore, the possibility of damaging the outer coating isminimized. However, the beam of the combiner's light output does notloose its high quality, since the single modes of respective SM fiberoutputs unlikely reach the outer boundary of the block, which otherwisewould lead to the increase of M² factor. Further, the structure of blockhas a flat downstream end, which looks away from the combiner. Theapplication of anti-reflecting material to such a flat face does notpose technological problems.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other details of the disclosure will become more readilyapparent from the specific description of the disclosure accompanied bythe following drawings, in which:

FIG. 1 illustrates the far-field divergence of a Gaussian-shaped beam.

FIG. 2 illustrates the dependence of the divergence from an MFD in SMfibers.

FIG. 3 illustrates the dependence of an MFD from a core size in SMfibers.

FIG. 4 illustrates a typical fiber combiner configured from multiple SMfiber lasers.

FIG. 5 is a cross-sectional view of the fiber combiner along lines V-Vof FIG. 4.

FIG. 6 is a cross-sectional view of the disclosed combiner.

FIG. 7 illustrates a process of manufacturing the disclosed combiner.

FIG. 8 graphically illustrates one of the steps of the disclosed processof FIG. 7.

FIGS. 9A and 9B illustrate the modification of mode in a SM fiber laseras the latter is processed in accordance with the disclose process.

FIG. 10 illustrates the actual view of the effective area of radiationemitted by the disclosed combiner.

FIG. 11 illustrates one of the embodiments of the disclosed combiner.

FIG. 12 illustrates still another embodiment of the disclosed combiner.

FIG. 13 illustrates the disclosed fiber laser system provided atermination block.

FIG. 14 illustrates one of the embodiments of the termination block ofFIG. 13.

FIG. 15 illustrates the other embodiment of the termination block ofFIG. 13.

SPECIFIC DESCRIPTION

Reference will now be made in detail to the disclosed combiner. Whereverpossible, same or similar reference numerals are used in the drawingsand the description to refer to the same or like parts or steps. Thedrawings are in simplified form and are far from precise scale.

The combiner manufactured and configured in accordance with thedisclosure is operative to radiate a combiner output which is resultedupon superimposition of a plurality of SM fiber outputs each having anoptimally low far-field divergence or numerical aperture and, thus,smallest possible M² for the combiner output.

FIG. 6 illustrates the end view of the disclosed high power fiber lasersystem 20 including a combiner 24. A plurality of SM fibers lasers 22,which are further referred to as SM fibers, have respective terminalends placed in parallel to one another which are further heated andstretched so that the claddings of respective SM fibers 22 collectivelydefine combiner 24 with a diameter Dbo, whereas each fiber 22 has a corediameter Dco. While the overall, odd or even number of SM fiber lasers22 may broadly vary and be limited only by technological restrictions,illustrated combiner 24 has seven SM fibers 22. In comparison to theprior art combiner 14 of FIG. 5, the dimensions of each fiber 22 and,therefore, the outer diameter of combiner 24 are substantially smallerthan the corresponding dimensions of combiner 14.

The terminal ends of respective SM fibers 22 may be arranged similarlyto that one of the shown prior art, i.e. multiple peripheral SM fiberlasers are disposed around the central SM fiber. The core 26 of eachfiber 22 is doped with rare-earth elements including, but not limitedto, erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium.

One of salient features of the disclosure relates to controllable radialreduction of combiner 24 and each of SM fibers 22. The main objective ofsuch a reduction is to provide each SM fiber 22 with core 26 having sucha maximally reduced diameter Dco that would allow for the optimallysmall NA of the fiber output. Such a configuration of individual SMfibers 22 would lead to the smallest possible M² factor of the combineroutput for a given number of SM fibers 22. The maximally reduced corediameter Dco of fiber 22 is a core diameter which each SM fiber 22 mayhave while still radiating a fiber output characterized by a Gaussianshape and maximally possible or optimal MFDo. The core diameter smallerthan the maximally reduced Dco and the MFD greater than optimal MFDocreate conditions under which the fiber outputs looses the Gaussianshape. In other words, a SM fiber with the geometry differing from thatone with the optimal core diameter Dco and, therefore, optimal NAo wouldemit an inferior fiber output. Once the fiber outputs of respective SMfibers 22 each are characterized by the optimally small NA, thecombiner's output beam has a minimally possible low far-field divergencewith smallest possible M² factor—the highest possible beam quality.

FIG. 7 illustrates a process for controllably reducing combiner 24 sothat the core diameter of each SM fiber 22 is reduced to the optimallyminimal core diameter (reference) Dco corresponding to a maximallypossible or optimal MFDo. Both of these optimal reference values, Dcoand MFDo, are empirically obtained values. Similar to the known art,terminal ends of respective SM fibers 22 are placed around the terminalend of the central fiber laser so as to form combiner 24 in a step 28.In step 30, combiner 24 is exposed to elevated temperatures and tensionforces tending to adiabatically stretch and, therefore, radially reducecombiner 24. For example, combiner 24, including seven SM fibers 22,each having a 125 μm outer diameter, may have optimal outer combinerdiameter Dbo radially minimized to 50, 60 or 70 μm.

FIGS. 8, 9A, 9B and 10 illustrate modifications occurring in singlefiber 22 during step 30 of FIG. 7. Referring to FIG. 9A, the fiberoutput of each individual SM fiber 22, (only two adjacent SM fiber laser22 are shown), has a Gaussian shape 38, and an MFDi corresponding to aninitial core diameter Dci of FIG. 8. The diameter Dci is not arbitrarilyshown on the curve. In practice, the core diameter Dci of each SM fiberlaser 22, as shown, is very convenient because of 1. substantialmode-matching with standard passive (undoped) fibers and, thus, lowlosses at splices, and 2. cut-off wavelengths occurring beforerespective operating wavelengths. However, as mentioned above, the MFDicorresponding to core diameter Dci may be small and, thus, thedivergence angle and numerical aperture may be large while the effectivearea of combined, but not tapered fibers may be large. Consequently,plurality of SM fibers 22 configured in accordance with theabove-disclosed parameters lead a poor quality combiner output ofcombiner 24 which has a broad far-field divergence and, thus, a high M²factor.

As combiner 24 radially reduces, so does initial core diameter Dci ofeach SM fiber 22. Looking at FIG. 8, it can be seen that the MFD of theoutput of SM fiber 22 first gradually decreases before this trend isreversed. The enlargement of MFD is explained by expansion of mode 38 ofeach SM fiber output gradually coupling out of core 26, as shown in FIG.9B, into the cladding of SM fiber 22. In accordance with the disclosedprocess, the expansion of mode 38 stops short off an outer boundary 39of the cladding of SM fiber laser 22 which defines a segment of theouter boundary OB of reduced combiner 24 as interstices between adjacentcladdings of respective fibers 22 get smaller during step 30. If themode 38 reaches outer boundary 39, the SM fiber output looses itsGaussian beam shape, and the overall quality of the combiner outputdeteriorates. On the other hand, it is desirable that the modes ofadjacent fiber laser outputs expand so as overlap with one anothersince, as modes 38 of respective SM fiber lasers move closer to oneanother. As a result, effective area 41 (FIG. 10) of the combineroutput, i.e., the inner boundary IB encircling the tips of the modes,tends to become smaller which, obviously, leads to the smaller. M²factor of the combiner output.

Returning to FIG. 7, in order to avoid the coupling of any single modeinto the boundary of the reduced combiner 24, the divergence angle (NA)of each individual SM fiber output and overall combiner output arecontinuously measured in step 32 by methods known to the artisan. Sincein SM fibers 22 the half-divergence angle corresponds to a NA, knowingthe latter a central processing unit (not shown) can calculate acorresponding MFDm of individual SM fiber output as indicated by step 34in accordance with the following equation:

$\begin{matrix}{{MFDm} = \frac{4\lambda}{\pi\; 2{NA}}} & (3)\end{matrix}$where λ is a known lasing wavelength, and NA (divergence half-angle) isa known NA of SM fiber. As readily realized by one of ordinary skills inthe art NAmeasured of the combiner output is the same as the NA of eachSM fiber 22 provided, of course, the fiber output preserves a Gaussianshape.

The MFDm of each SM fiber 22 is continuously determined and compared tothe optimal diameter MFDo in step 34. Once the measured MFDm issubstantially the same as MFDo, the process continues as disclosedimmediately above.

The beam quality factor of the combiner output is determined in as

$M^{2} = \frac{NAmeasured}{NAideal}$Since the NAmeasured of the combiner output is substantially the same asthat one of SM fiber 22 and, thus, known, it is necessary to determineNAideal. Only the MFD, as a concept, cannot be applied to the combineroutput since the latter is a MM beam. Accordingly, instead of measuringthe MFD, the effective area EA of the combiner output is determined instep 35 asEA≈MFDo×n  (5)wherein MFDo is experimentally selected, and n is a number of fused anddiametrically aligned SM fibers 22 (for example, three of FIG. 10)delimited by the outer boundary 39 (FIG. 10) of the combiner output.Once the EA is determined, the NAideal can be determined in step 36 asfollows:

$\begin{matrix}{{NAideal} = \frac{4\lambda}{\pi\; 2{EA}}} & (6)\end{matrix}$

Having determined NA ideal for the calculated EA, the beam qualityfactor is determined in step 44 according to equation 4. Whiledetermining the Mm² factor, it is compared to the desired factor of thecombiner output, which is determined based on the selected optimalvalues of the Dco and MFDo of SM fiber.

If, however, the quality of the combiner output is not sufficientlyhigh, which may happen even if the optimal values of SM fiber outputshave been reached, the process continues as indicated by step 46. The M²of the combiner output still may be high because of several factors.One, of course, the NA and, therefore, MFDo of each SM fiber output maynot be optimal and, thus, may be further enlarged provided that the SMoutput preserves a Gaussian shape. Typically, however, a high M² factoris a consequence of excessively large MFD of SM fiber outputs which havereached the outer boundary 39 (FIG. 10) of the combiner output. Thereduction of the beam quality factor can be realized by structuresillustrated in FIGS. 11 and 12.

To minimize a possibility of coupling of individual modes into theboundary of the combiner output, the boundary should be expanded. FIG.11 diagrammatically illustrates a sleeve 48 with a diameter D receivingcombiner 24 so as to be stretched therewith. As a result, sleeve 48,when heated and stretched, adds an extra layer to the boundary of thecombiner's radiation. FIG. 12 shows a plurality of empty fibers or cords50 defining diameter D and surrounding combiner 24, showndiagrammatically in phantom lines. The cords 50 function similarly tosleeve 48 of FIG. 11 and, thus, upon stretching, expand the boundary ofthe radiation.

Returning to FIG. 7, alternatively, instead of steps 32, 34, 36 and 37,step 40 provides for measuring a coefficient of core diameterattenuation. The coefficient of attenuation of core diameter 26 of SMfiber 22 is calculated as

${k = \frac{Dci}{Dcm}},$where Dcm is a measured diameter. Then the coefficient of attenuation iscontinuously compared to an experimentally determined reference corediameter Dco in step 42 corresponding to maximum reduction of corediameter Dco beyond which the fiber laser's output ceases to have aGaussian shape. After the desired coefficient has been reached, theprocess proceeds as disclosed above by first determining the combineroutput beam quality factor M² in step 44 and subsequently, if needed,modifying it in step 46 in accordance with the techniques disclosed inconjunction with FIGS. 12 and 13.

Numerous experiments involving disclosed combiner 24 consisting of sevenSM fiber lasers 22 show that the M² factor can be reduced from 7, whichis a result of the configuration shown in FIG. 5, to 5 and even to 3.Considering that the combiner output can be as powerful as tens of kWs,the beam quality of combiner 24 is extremely high.

Of course, other methods of configuring combiner 24 with the desiredbeam quality factor can be used. For example, since the reduction ofcore diameter Dc and the outer diameter Dcl of the cladding of SM fiberlaser 22 is proportional, the optimal coefficient of reduction Krdetermined as

$\begin{matrix}{{{kr} = \frac{Dclo}{Dco}},} & (7)\end{matrix}$where Dclo is an experimentally established optimal diameter of thecladding. Once the optimal coefficient of reduction is achieved, theprocess may be stopped.

FIG. 13 shows a further aspect of the disclosure. As has been disclosedabove, the power of the combiner output of the disclosed fiber lasersystem may reach tens of kWs. The enormous power of the combiner outputposes two distinct problems. First, the backreflection from an endsurface creates a backreflected light which, propagating back intocombiner 24, may burn everything along its path. Second, anantireflection layer of material preventing the propagation of thebackreflected light is difficult to directly apply to the output fiberof the combiner, because its outer surface is somewhat uneven anddifficult to polish. Even if the AR coating is applied, it may bedamaged because of a high power density.

The solution to both of the above-discussed problems includes the use ofa coreless quartz termination block 52 fused to the output end ofcombiner 24. The combiner output expands within block 52 similar to theexpansion of the beam in free space. Accordingly, separate fiber outputseach preserve its Gaussian shape and spatial relationship with oneanother, i.e., the modes of respective adjacent fiber outputs overlapone another like they would in free space without reaching the outerboundary of block 52. As a consequence, the combiner output or radiationhas the same characteristics as obtained while using fiber laser system20 of FIG. 6. However, a face 54 of block 52 is flat and relativelylarge which provides for an easy application of antireflection layer.Accordingly, the use of block 52 simplifies the technological processand provides protection of combiner 24 from the end surfacebackreflection.

FIGS. 14 and 15 illustrate respective modifications of the geometry ofblock 52. It is desirable that the opposing ends of respective combiner24 and block 52 be comparably scaled which simplifies the splicing ofthese components. The opposite, downstream end of block 52 should bepreferably greater than the upstream end thereof because, while thecombiner output diverges, it is necessary to prevent its coupling intothe boundary of the block. FIG. 14 illustrates block 52 having afrustoconical-shaped cross-section with opposite longitudinal sides ofthe block diverge from one another along the direction of propagation offorward light. Alternatively, FIG. 15 also illustrates block 52 with afrustoconically-shaped cross-section, but the longitudinal sides eachhave a stepwise configuration with the upstream end of block 52, whichis coupleable to the opposing end of combiner 24, being small enough tolosslessly couple to the combiner's output. Other shapes including, forexample, cylindrical and polygonal shapes are contemplated within thescope of the disclosure.

Although there has been illustrated and described in specific detail andstructure of operations, it is understood that the same were forpurposes of illustration and that changes and modifications may be madereadily therein by those skilled in the art without departing of thescope of this disclosure.

The invention claimed:
 1. A method for configuring a fiber combinercomprising: configuring a plurality of single mode (“SM”) fibers eachwith an optimal numerical aperture (NA); coupling the SM fibers to oneanother; emitting outputs by respective SM fibers; combining the outputstogether in a multimode (MM) combiner output so that the combiner outputhas a minimally possible beam quality factor (M²), wherein the optimalNAs of SM fibers each is so selected that any NA lower than the optimalNA results in the M² of MM combiner output higher than the minimallypossible M².
 2. The method of claim 1, wherein the configuring of eachSM fiber with the optimal NA includes structuring a core of the SM fiberwith an optimal core diameter supporting a single mode with a largestpossible mode field diameter (MFD), the largest MFD being a thresholdabove which the SM fiber output looses a Gaussian shape and the M² ofthe MM combiner output is higher than the minimally possible M² factor.3. The method of claim 2, wherein the coupling of the SM includes fusingthe SM fibers so that the MM combiner has an outer diameter providedwith an outer boundary which is defined by fused claddings of respectiveSM fiber lasers.
 4. The method of claim 3, wherein the fusing of the SMfibers terminates before the modes come in contact with the outerboundary of the MM combiner output.
 5. The method of claim 2 furthercomprising forming a sleeve around the SM fibers, and fusing the sleevewith the SM fibers so that the modes do not touch the fused sleeve whichdefines the outer boundary of the MM combiner output.
 6. The method ofclaim 2 further comprising providing a plurality of coreless fibersaround the SM fibers, and fusing the coreless fibers with the SM fibersso that the coreless fibers form an outer boundary of the MM combineroutput spaced radially from the SM modes.
 7. The method of claim 1,wherein configuring of the SM fiber includes selecting an odd number ofthe SM fibers or an even number of the SM fibers.
 8. The method of claim1, wherein the optimal NA corresponds to a minimally possible far-fielddivergence of⁻the combiner output.
 9. The method of claim 1 furthercomprising configuring a coreless termination block from quartz, andfusing the block to an output of a combiner so as to preserve thecombiner output with the minimally possible M² factor.
 10. The method ofclaim 9, wherein the configuring of the block includes providing theblock with one of cylindrical, polygonal or frustoconical cross-section.11. The method of claim 9, wherein the configuring of the block furtherincludes providing a peripheral wall of the block with a stepwisestructure.
 12. The method of claim 10 further comprising covering anfree end of the termination block with an anti-reflection coating. 13.The method of claim 1, wherein the M² of the combiner vary between 3 and5.